1. Technical Field
This invention relates generally to level-estimation in multi-level cell memory, and more particularly to methods and apparatus for determining reference signal levels corresponding to respective levels of multi-level memory cells. These reference signal levels provide the basis for detecting stored codewords in memory read operations.
2. Description of the Related Art
In multi-level cell (MLC) memory, the fundamental storage unit (the “cell”) can be set to q>2 different states, or “levels”, permitting storage of more than one bit per memory cell. Detection of stored data relies on identifying the different cell levels on readback. In solid-state memory such as flash memory and phase change memory (PCM), for example, the different cell levels exhibit different electrical resistance characteristics which can be detected via current or voltage measurements on the cells. When writing information in MLC memory, each cell can be used to store a qary symbol with each of the q possible symbol values being represented by a different cell level. On readout of multi-level cells, the read signal level is compared with a set of reference signal levels indicative of the q cell levels in order to determine which level each cell is set to and thus detect the stored symbol value.